1. Field of the Invention
The invention relates to a backlight driving circuit for a liquid crystal display (LCD) device, and more particularly, to an arrangement in a backlight driving circuit to improve spatial utilization and temperature stability by arranging inverters for driving fluorescent lamps in a diagonal direction.
2. Description of the Related Art
With rapid development of information communication fields, the importance of displaying desired information has dramatically increased. Cathode ray tubes (CRTs) have recently found common use as display devices in televisions and computer monitors because of their ability to display various colors with high luminance. However, CRTs are relatively large and cannot adequately satisfy present demands for display applications that require reduced weight, portability, low power consumption, increased screen size and high resolution. Flat panel displays have accordingly been developed for use as monitors for computers, spacecraft, and aircraft.
Various flat panel displays that are in use include, for example, a liquid crystal display (LCD) device, an electro-luminescent display (ELD), a field emission display (FED), and a plasma display panel (PDP). Currently, practical application of the flat panel displays requires high luminance, great efficiency, high resolution, rapid response time, low driving voltage, low power consumption, low manufacturing cost and natural color display characteristics. Among the flat panel displays, the LCD device has attracted great attention by having portability and endurance as well as the aforementioned characteristics required for the flat panel displays.
The LCD device is a display device exploiting the optical anisotropy of liquid crystals. That is, when light irradiates on the liquid crystal having polarizing characteristics according to an applied voltage state, light transmittance is controlled by the alignment state of the liquid crystal, thereby displaying a picture image. However, the LCD device in and of itself does not emit light, and the LCD device therefore requires an additional light source. One such LCD device is a reflective type LCD device. A reflective type LCD device uses ambient light but has limitations due to the environmental problems such as, e.g., low ambient light levels. As a result, a transmitting type LCD device having an additional light source such as a backlight has been developed. For instance, light sources such as electro-luminescence (EL), a light-emitting diode (LED), a cold cathode fluorescent lamp (CCFL) and a hot cathode fluorescent lamp (HCFL) are used for the backlight of the transmitting type LCD device. Of these, the cold cathode fluorescent lamp (CCFL) is most widely used for the backlight because the CCFL is thin and has low power consumption.
The backlight of the transmitting type LCD device classifies into a direct type and an edge type according to the location of the fluorescent lamp. In the edge type backlight, a cylindrical fluorescent lamp is formed at one side of the LCD panel, and a transparent light-guiding plate is formed to transmit the light emitted from the fluorescent lamp to an entire surface of the LCD panel. The edge type backlight has the problem of low luminance. Also, optical design and processing technology for the light-guiding plate are required to obtain uniform luminance.
Meanwhile, the direct type backlight is suitable for a large sized LCD device of 20 inches or more, in which multiple fluorescent lamps are arranged in one direction below a light-diffusion plate to directly illuminate an entire surface of the LCD panel with light. That is, a direct type backlight unit having great light efficiency finds common use for the large size LCD devices requiring high luminance. However, the direct type is problematic in that a silhouette of the fluorescent lamp may reflect on the LCD panel. Thus, a predetermined interval must be maintained between the fluorescent lamp and the LCD panel, and it is thus hard to obtain a thin profile in an LCD device having a direct type backlight unit. As the panel becomes larger, the size of the light-emitting surface of the backlight increases. With a large-size direct type backlight, an appropriate thickness of a light-scattering means is required. If the thickness of the light-scattering means is not appropriately thin, the light-emitting surface is not flat.
Despite this, the direct type backlight finds use in an LCD device requiring high luminance, and an edge type backlight unit finds general use in relatively small size LCD devices such as monitors of laptop computers and desktop computers. With the trend towards increasingly large sized LCD panels, the direct type backlight is actively developed by forming multiple fluorescent lamps under a screen, or by disposing one bent fluorescent lamp, thereby obtaining a high luminance backlight.
FIG. 1 shows a perspective view illustrating a direct type backlight according to the related art, and FIG. 2 schematically illustrates a fluorescent lamp. As shown in FIG. 1, the direct type backlight according to the related art includes multiple fluorescent lamps 1, an outer case 3, and a light-scattering means 5. The fluorescent lamps 1 are arranged at fixed intervals in one direction, and the outer case 3 fixes the plurality of fluorescent lamps for maintaining the fixed intervals. The light-scattering means 5 is provided above the fluorescent lamps 1. The light-scattering means 5 prevents the silhouette of the fluorescent lamps 1 from being reflected on the display surface of the LCD panel (not shown), and provides a light source with uniform luminance. For improving the light-scattering effect, the light-scattering means 5 is composed of a diffusion plate 5a and multiple diffusion sheets 5b and 5c. Also, a reflecting plate 7 is provided inside the outer case 3 for concentrating the light emitted from the fluorescent lamps 1 to the display part of the LCD panel, thereby improving light efficiency. Also, FIG. 2 shows that the fluorescent lamps 1 are respectively fixed to both sides of the outer case 3. Each fluorescent lamp 1 is a cold cathode fluorescent lamp 1, which is charged with discharge gas. Each fluorescent lamp 1 includes electrodes 2a and 2b for receiving external power (not shown), and wires 9a and 9b connected to the electrodes 2a and 2b. The wires 9a and 9b are provided A.C. voltage 4 and connect to a driving circuit by an additional inverter (20 in FIG. 3.). Each fluorescent lamp 1 thus requires an additional inverter. Meanwhile, the backlight driving circuit for driving the plurality of fluorescent lamps 1 has the multiple inverter circuits, and is provided at the rear of the backlight.
FIG. 3 shows a circuit diagram illustrating a driving circuit provided at the rear of a related art backlight. FIG. 4 shows a detail view illustrating a low-voltage part of FIG. 3. As shown in FIG. 3, the driving circuit of the related art backlight includes a high-voltage part 21, a low-voltage part 23, and a connection part 25. The high-voltage part 21 is formed at one portion of a rear side of an LCD panel to apply an A.C. high voltage to a first terminal of each fluorescent lamp (‘1’ of FIG. 1). The low-voltage part 23 is formed at the other portion of the rear side of the LCD panel to apply a lower electric potential (as compared to that of the high-voltage part 21) to a second terminal of each fluorescent lamp (‘1’ of FIG. 1). The connection part 25 is formed to connect the low-voltage part 23 to a feedback terminal of the high-voltage part 21. The high-voltage part 21 includes multiple inverter circuits 20 for converting a D.C. voltage to an A.C. voltage to drive corresponding fluorescent lamps (‘1’ of FIG. 1). A group of first connectors 32a each connect connecting the first terminal of the fluorescent lamp 1 to the inverter circuit 20. The low-voltage part 23 includes a group of second connectors 32b, and each of the second connector 32b connect to the second terminal of the fluorescent lamp 1. Also, the connection part 25 includes insulated wires corresponding to the number of fluorescent lamps (‘1’ of FIG. 2), and first and second feedback connectors 22a and 22b electrically connect the high-voltage part 21 to the low-voltage part 23. Also, as shown in FIG. 1, the fluorescent lamps are arranged in parallel to the horizontal direction of the LCD panel. Also, the power supplying wires 9a and 9b are formed at both ends of each fluorescent lamp 1, and are connected by the first connector 32a of the high-voltage part 21 and the second connector 32b of the low-voltage part 23.
FIG. 3 shows that the connection part 25 may be formed as a single wire, or multiple wires corresponding to the number of fluorescent lamps and according to the control method of the fluorescent lamps 1. The voltage or current of the low-voltage part input by feedback from the inverter circuit 20 controls the current of the fluorescent lamps 1. If a single wire is used, problems may occur due to different characteristics of the respective fluorescent lamps. If a number of wires are used, it is possible to control the fluorescent lamps in due consideration of the impedance of the respective fluorescent lamps 1. As a result, deflection of the current decreases among the multiple fluorescent lamps 1, thereby providing uniform luminance by decreasing the difference in luminance among the fluorescent lamps 1. That is, as shown in FIG. 4, the low-voltage part 23 of the related art backlight includes multiple second connectors 32b, each connected to the power supplying wire (9 or 9a) of each fluorescent lamp. Multiple power source wires 26 also respectively connect to the second connectors 32b and the second feedback connector 22b to collect the multiple power source wires 26 on a PCB (printed circuit board). Each of the first and second connectors 32a and 32b may connect to the power supplying wires of two fluorescent lamps.
FIG. 5 shows a related art circuit diagram schematically illustrating an inverter circuit of a backlight. Each inverter circuit 20 includes first and second switching devices Q1 and Q2, and a high voltage Transformer T1. The first and second switching devices Q1 and Q2 output a driving voltage Vcc1 to a high voltage Transformer T1 by alternately switching the driving voltage Vcc1. The high voltage Transformer T1 includes a primary coil and a secondary coil, in which the primary coil receives the driving voltage Vcc1 from the switching devices Q1 and Q2, and the secondary coil outputs a high voltage according to a winding ratio (n1:n2) of the primary and secondary coils. The first and second switching devices Q1 and Q2 are switched by output of a third coil (n3) for inducing the low voltage from the secondary coil (n2). Herein, L1 is a line filter, R1-R3 are resistors, C1-C3 are condensers (capacitors), and D1 is a diode. As mentioned above, the inverter circuit 20 includes the high voltage Transformer T1. That is, even though the inverter circuit 20 is formed on the PCB, it requires a large space.
The operation of the inverter circuit in the backlight for the related art LCD will be described as follows. First, the inverter driving voltage Vcc1 is input through the line filter L1, and the first and second switching devices Q1 and Q2 alternately switches the inverter driving voltage Vcc1 by push-pull operation, thereby outputting the inverter driving voltage Vcc1 applied to a collector to the primary side of the Transformer T1. Then, the Transformer T1 outputs the voltage induced to the primary side n1 to the secondary side n2 according to the winding ratio of n1 to n2, and outputs the A.C. high voltage to the fluorescent lamp 1 through the first connector 32a. By the A.C. high voltage output from the high voltage Transformer T1, the current flows in the fluorescent lamps 1 through the first and second connectors 32a and 32b. At this time, the voltage corresponding to resistor capacity R3 and the current flowing in the fluorescent lamp 1 generates in the second connector 32b. That is, the voltage corresponding to current×resistance R3 of the fluorescent lamp 1 is caught by the second connector 32b. 
However, the backlight driving circuit according to the related art has many disadvantages, including those discussed below.
As LCD devices become larger, it becomes necessary to increase the length of the fluorescent lamp. Thus, one needs to increase the capacity and size of the components of the inverter circuit. In the related art backlight driving circuit shown in FIG. 3, the high-voltage part having the inverter circuit is formed at one portion of the rear side of the LCD module, and the low-voltage part is formed at the other portion of the LCD module. Accordingly, the size of the PCB forming the high-voltage part becomes greater than that of the vertical size of the LCD module due to the size of the high voltage transformer of the inverter circuit, thereby increasing the outer size of the LCD device.
Also, since the high-voltage part having the inverter circuit is formed at one portion of the rear side of the LCD module, and the low-voltage part is formed at the other portion thereof, it becomes difficult to obtain a uniform temperature distribution in the portions forming the high-voltage part and the low-voltage part, thereby shortening the lifespan of the fluorescent lamp due to deflection of the gas therein.
FIG. 6 shows a temperature distribution graph of an LCD device having a related art arrangement of a backlight driving circuit. The high-voltage part includes an inverter circuit containing a high voltage transformer, whereby the high-voltage part emits relatively greater heat than that of the low-voltage part. Also, the high-voltage part absorbs heat generated from the fluorescent lamp. Accordingly, the temperature difference between the high-voltage part and the low-voltage part becomes large. FIG. 6 illustrates the result of a temperature gradient at a rear side of a bottom cover in the LCD device when the environmental atmosphere is at a temperature of 28° C. Accordingly, if the fluorescent lamp is driven for a long time, gas such as mercury is deflected, thereby shortening the lifespan of the fluorescent lamp.